The present invention relates to a method for transmitting time-critical data in a packet-oriented data transmission network having a number of communications devices connected to one another, and a communications equipment unit for carrying out the method.
The present invention is concerned with a communications system having a number of spatially distributed communications equipment units. The individual communications equipment units are connected to one another via a packet-oriented data transmission network, e.g., a local area network (LAN) and/or a wide-area network (WAN). Furthermore, the communications equipment units are connected to a public communications network. Data is transmitted over the data transmission network via of an asynchronous packet-oriented transmission protocol, such as the Internet Protocol (IP).
Time-critical data, particularly voice data, to be transmitted, inter alia, between the communications equipment units, is transmitted over the data transmission network asynchronously, for instance, after having been packaged into IP packets. The term frequently used to describe this procedure is “Voice Over IP”, also known as VoIP. Individual sampled values of the signal to be transferred (i.e., voice data to be transferred) are packaged into what are referred to as “frames”. A number of frames are in turn packaged into a data packet and transferred over the data transmission network to the respective receiver (i.e., to the respective communications equipment unit).
Some of the known packet-oriented data transmission networks, such as an IP-oriented network, cannot ensure the service quality which is necessary for the transmission of time-critical data (i.e., “Quality of Service”, also known as QoS). However, when time-critical data, such as voice data, are transmitted, the effects of a number of QoS parameters, such as packet transit times, jitter, the packet throughput rate and packet loss rate, are of decisive importance for the transmission quality of the time-critical data.
The known standard H.323 of the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) contains technical specifications for audio and video communication over packet-oriented data transmission networks which do not provide definite service quality. The standard H.225.0, which is associated with the standard H.323 of the ITU-T, specifies, inter alia, mechanisms for packaging, synchronizing and maintaining service quality. The service quality is maintained by taking into account the packet loss rate, whereby bit errors which can lead to a packet loss are also taken into account, and by taking into account the delays/transit times within a link. The core mechanisms for achieving the service quality are always defined or effective in a transmitter device (i.e., from a communications equipment unit to the data transmission network).
Within the standard H.323, the Real Time Protocol or Real Time Transfer Protocol (RTP) and the Real Time Control Protocol (RTCP) are defined. These protocols are used for real-time-critical transmission of data, such as audio and video data, over a packet-oriented data transmission network. In order to reduce a packet loss rate, overloading of the data transmission network is monitored via an RTCP component of the respective data packets, the transmission load being reduced if overloading occurs. Furthermore, disruptive echoes from one communications device to the data transmission network, which arise as a result of a prolonged transit time of data packets, are eliminated via echo compensation.
In the prior art, the following measures for improving the quality of time-critical data to be transmitted, particularly voice data, are implemented in communications equipment units with an integrated H.323 functionality. According to the standard H.225.0, in order to reduce the packet loss rate, the packet transmission rate is reduced in the transmission direction if there is an overload of the data transmission network. This is done by increasing the number of frames per RTP packet. The control is performed via RTCP data. The packet rate is also reduced via what is referred to as “Silence Compression/Suppression” (i.e., by virtue of the fact that data is not transmitted over the data transmission network in a pause in speech).
For echo compensation, an “echo canceler” is implemented, for instance, in accordance with the standard G.165 of the ITU-T, for each link, it being possible to activate or deactivate the echo canceler according to requirements.
In addition, in the event of a packet loss in a receiver device (i.e., from the data transmission network to the communications equipment unit), a noise packet (if appropriate even in attenuated form) is output.
An object of the present invention is, therefore, to provide a method for transmitting time-critical data in a packet-oriented data transmission network and a communications equipment unit for carrying out this method, the method and equipment unit optimizing the quality of a link over the packet-oriented data transmission network.